(1) Field of the Invention
The present invention relates to film-forming, oleophilic photopolymerizable compositions, to the use of such compositions in formation of pre-sensitized photosensitive articles and to negative-working lithographic printing plates so formed. More particularly, the present invention relates to negative-working photopolymerizable, film-forming, oleophilic compositions which are soluble in organic solvents and which are based on specific epoxy resins and specific photosensitive diazo compounds as cross-linking agents for the epoxy resins and to presensitized photosensitive articles, especially negative-working lithographic plates, developable with acidified water using a layer of the film-forming oleophilic photopolymerizable compositions as the photosensitive layer.
(2) Discussion of the Prior Art
The lithographic printing technique for which the compositions of this invention are most particularly intended, is based on the principal of non-miscibility of greasy inks and water and by the formation on a suitable support of hydrophilic, oleophobic zones (i.e. retaining water and rejecting greasy ink) and hydrophobic, inkophilic zones (i.e. retaining greasy ink and rejecting water). After wetting the support with water, the greasy ink is applied to the entire surface and is held at the inkophilic zones but is rejected by the wet hydrophilic zones.
Therefore, lithographic printing plates are formed by coating a photosensitive layer on a suitable support. The photosensitive layers that are used in lithography are generally of two types:
positive working layer in which the zones unexposed to light are kept as image zones after development; and
negative working layer in which the unexposed zones are eliminated by development and the exposed zones form a hard printing pattern.
The present invention is particularly concerned with negative working photosensitive compositions and the photosensitive articles and lithographic plates using such negative working photosensitive compositions coated on an underlying support which in the case of lithography is a hydrophilic support. In the case of the negative working lithographic plates, the plate is exposed to ultraviolet light through a negative having transparent zones and opaque zones. The photosensitive composition is polymerized and hardened in zones corresponding to the transparent zones of the negative. The unexposed zones, which correspond to the opaque zones of the negative, are then removed by a developer which does not attack the exposed zones thereby baring the underlying hydrophilic support. The exposed zones are the "image" zones which receive the greasy ink, while the unexposed zones are the "non-image" zones.
At present, most lithographic plates currently used in printing are of presensitized type, i.e., coated with a photosensitive layer and ready to be exposed through an original (generally a transparent support partially covered with opaque zones) to be reproduced.
It is apparent that for negative working lithographic plates, especially printing plates in which the developed plate will be subjected to mechanical stresses and which are typically used for making large numbers of copies, often exceeding 50,000 or even 100,000 copies, it is essential to use photopolymerizable resins which have exceptional hardness and exceptional tenacity. It is known that epoxy resins are particularly well suited to this purpose.
It is known from U.S. Pat. No. 3,205,157 that some diazonium salts such as aryl diazonium fluoroborates mixed with epoxy resins are able, under the action of light, to release a Lewis acid that catalyzes the polymerization reaction of the epoxy resin with itself. For this purpose, the best results are obtained by using 1 to 10 parts of diazonium salt per 100 parts of epoxy resin. However, these mixtures can be used only to make printed circuits and, in a general way, for protective varnishes, but they are not used in the field of lithographic plates. Actually, as is explained in U.S. Pat. No. 3,708,296, these photosensitive compositions, after polymerization, have only a slight receptivity to greasy inks, this property obviously being critical in the graphic arts field. Further, these compositions have other drawbacks such as a very short life due to the chemical instability of the diazonium salts used.
U.S. Pat. No. 3,930,856 also teaches that the presence of an amino group in the diazonium salt should be avoided if it is desired to catalyze the polmerization of the epoxy resins because the freed Lewis acid tends to combine with this amino group and no hardening of the epoxy resin is then noted.
French Pat. No. 2,011,413, to which U.S. Pat. No. 4,093,465 corresponds, describes negative photosensitive compositions which are soluble in organic solvents and which are formed as a mixture of a photosensitive and photohardenable product made up of a photosensitive, water-soluble condensation product of a diazonium compound and an organic condensation agent which is reacted with an organic coupling agent which makes this product insoluble in water, and an oleophilic, film-forming resin soluble in organic solvents. Among the photosensitive diazonium compounds disclosed in these patents are the diazo aromatic compounds and especially p-diazo-diphenyl-amine and its derivatives, condensed with organic condensation agents having reactive carbonyl groups such as aldehydes and acetals and particularly formaldehyde and paraformaldehyde.
The synthetic resins to be mixed with these products are described to include substantially any of the resins which are known to be film-forming, oleophilic and insoluble in water. By way of non-limiting examples the references mention epoxy, phenolic, acrylic, polyamide, polystyrene, polyvinyl chloride, polyvinyl acetate, polyester, polyurethane resins.
It is further described in this French and corresponding U.S. Pat. that the photopolymerizable compositions can be used for making lithographic plates which are developable with aqueous or substantially aqueous solutions.
It is known to be a great advantage to users of lithographic plates that the plates be as simple as possible to use, and therefore development with aqueous solutions for this purpose is highly desirable.
However, at the present time, negative working plates which are developable with water and which have a very long life have not been made practically available on a commercial scale.
U.S. Pat. No. 3,396,019 describes presensitized plates having a support of aluminum treated with a phosphonic acid or one of its derivatives, which is coated with a layer of oleophilic resin which is insoluble in water and a water-soluble condensation product of a diazonium compound, such as paradiazodiphenylamine condensed with formaldehyde in an acid medium. This reference also mentions epoxy resins among the resins that are suitable for making lithograhic plates. However, the plates described in this patent have numerous drawbacks. First, the photopolymerizable composition as described in the embodiments of this patent is very difficult to make in practice. Second, the medium in which the mixture of the oleophilic resin and water-soluble condensation product is prepared is a partially aqueous medium since the diazonium compound used is soluble only in an aqueous medium. Since epoxy resins have a very poor affinity for water it is difficult to use the epoxy resins in such systems.
Furthermore, in making the compositions as described in this patent, it has been found that the compositions precipitate very easily from the acid medium. Still further, these compositions are not film-forming and during coating in thin layers on a support, it has been found that separation of the constituents of the composition occurs, with the diazonium compound separating from the resin. Accordingly, these compositions are not industrially acceptable.
It is also known from U.S. Pat. No. 3,794,576 that monomer or prepolymer epoxy resins, which are fluid at ambient temperature, can be polymerized with from about 0.1 to 5 weight percent of certain diazonium salts such as hexafluorophosphates, tetrafluoroborates, tetrachloroferrates, hexachlorostannates, hexafluoro arsenates, hexafluoroantimonates, etc. which decompose to provide a Lewis acid upon exposure to energy. These compositions can be used to make images with the unexposed portions being removed with suitable solvents.